Shift control device for automatic transmission and control method thereof

ABSTRACT

An automatic transmission calculates a current thermal load state of the frictional element, predicts (S 24 , S 31 ), prior to the start of the shift, a heat generation amount of the frictional element if the shift is performed in the first shift mode, predicts (S 25 , S 32 ) a thermal load state of the frictional element upon shift completion on the basis of the current thermal load state of the frictional element and the predicted heat generation amount, and either performs (S 28 , S 38 ) the shift in a second shift mode, in which a heat generation amount is lower than that of the first shift mode, or prohibits (S 39 ) the shift when the predicted thermal load state upon shift completion is inside a predetermined region.

FIELD OF THE INVENTION

This invention relates to a shift control device for an automatictransmission.

BACKGROUND OF THE INVENTION

In a typical conventional automatic transmission for an automobile, therotation of an engine is input via a torque converter, varied in speedby a shift mechanism having a plurality of planetary gears, and outputto a drive shaft or a propeller shaft (the axle side).

The shift mechanism of this type of automatic transmission executes ashift by transmitting the rotation of an input shaft to a specific gearor carrier of the planetary gear in accordance with a shift position andtransmitting the rotation of the specific gear or carrier to an outputshaft appropriately. The shift mechanism also comprises a plurality offrictional elements such as clutches and brakes to converge the rotationof the specific gear or carrier appropriately during the shift, andperforms a predetermined shift by switching a torque transmission pathin accordance with engagement and disengagement combinations of thefrictional elements. Hydraulic clutches and brakes, the engagement stateof which is controlled through the supply and discharge of oil pressure,are typically employed as the frictional elements.

If a vehicle travels in the vicinity of a boundary region of a vehicletraveling condition when performing a predetermined shift in aconventional automatic transmission, the selected gear position may varysuch that the shift is repeated. For example, when performing a 3-4shift from a third speed to a fourth speed, the 3-4 shift from the thirdspeed to the fourth speed and a 4-3 shift from the fourth speed to thethird speed are repeated such that the gear position varies continuouslyfrom three to four to three to four and so on.

When shifts are performed continuously in this manner, the samefrictional elements are repeatedly engaged and disengaged over a longtime period, and therefore the thermal load applied to the frictionalelements increases (the temperature increases). As a result, the burnsmay occur on the frictional elements, leading to eventual burnout. Itshould be noted that in this specification, “thermal load” is used tomean “temperature” or “heat generation”.

In response to this problem, JP3402220B, published by the Japan PatentOffice, discloses a technique using a timer. More specifically, a timeris counted down during a continuous shift, and when the timer valuereaches a predetermined value, subsequent shifts are prohibited,assuming that the thermal load state (temperature) of the frictionalelement has reached a burnout temperature. When the continuous shiftends before the timer value reaches the predetermined value, the timeris counted up on a fixed gradient, assuming that heat radiation isunderway.

Hence, when the continuous shift resumes immediately after the end ofthe continuous shift, countdown of the timer value begins from a smallervalue than an initial value, and therefore control is executed takinginto consideration the amount of accumulated heat in the frictionalelement.

SUMMARY OF THE INVENTION

However, in the conventional technique described above, only time isused as a parameter, regardless of the type of shift and input torque,and the type of the next shift is not taken into account. Thepredetermined value of the timer value at which shifts are prohibited isset such that the frictional element is not damaged, irrespective of thetype of the next shift. In other words, the predetermined value of thetimer value is set at a value having a sufficient margin in relation toa temperature at which damage actually occurs, thereby ensuring that thefrictional element is not damaged even if a shift that generates amaximum heat generation amount is performed. Therefore, even when adetermined shift would not generate a large amount of heat and thefrictional element would not reach the damage temperature if the shiftwas performed, the shift is prohibited, and as a result, drivabilitydeteriorates.

It is an object of this invention to prevent drivability fromdeteriorating by improving shift tolerance.

In order to achieve the above object, this invention provides anautomatic transmission that comprises a shift mechanism that executes ashift from a current gear position to a target gear position by engagingor disengaging a plurality of frictional elements selectively, a shiftcontrol unit which performs the shift in a first shift mode, a currentthermal load calculating unit which calculates a current thermal loadstate of the frictional element, a first heat generation amountpredicting unit which predicts, prior to the start of the shift, a heatgeneration amount of the frictional element if the shift is performed inthe first shift mode, and a first thermal load predicting unit whichpredicts a thermal load state of the frictional element upon shiftcompletion if the shift is performed in the first shift mode on thebasis of the current thermal load state of the frictional element andthe heat generation amount predicted by the first heat generation amountpredicting unit. The shift control unit either performs the shift in asecond shift mode, in which a heat generation amount is lower than thatof the first shift mode, or prohibits the shift when the thermal loadstate upon shift completion predicted by the first thermal loadpredicting unit is inside a predetermined region.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pattern diagram showing the constitution of a shift controldevice for an automatic transmission according to an embodiment.

FIG. 2 is a skeleton diagram showing the structure of the automatictransmission according to this embodiment.

FIG. 3 is a view showing the engagement states of frictional elements ineach gear position of the shift control device for an automatictransmission according to this embodiment.

FIG. 4 is a view showing a shift map of the shift control device for anautomatic transmission according to this embodiment.

FIG. 5 is a block diagram showing control of the shift control devicefor an automatic transmission according to this embodiment.

FIG. 6 is a view illustrating a clutch temperature initial value in theshift control device for an automatic transmission according to thisembodiment.

FIG. 7 is a view illustrating a clutch temperature characteristic in theshift control device for an automatic transmission according to thisembodiment.

FIG. 8 is a view illustrating a reset determination timer of the shiftcontrol device for an automatic transmission according to thisembodiment.

FIG. 9 is a time chart of a PYUP shift.

FIG. 10 is a time chart of a PYDOWN shift.

FIG. 11 is a flowchart showing clutch temperature calculation control inthe shift control device for an automatic transmission according to thisembodiment.

FIG. 12 is a flowchart showing control for calculating a heat radiationamount during engagement.

FIG. 13 is a flowchart showing shift control in the shift control devicefor an automatic transmission according to this embodiment.

FIG. 14 is a flowchart showing shift control in the shift control devicefor an automatic transmission according to this embodiment.

FIG. 15 is a map showing an allowable number of continuous change-mindshifts.

FIG. 16 is a flowchart showing control for calculating a predictedtemperature during an UP shift.

FIG. 17 is a flowchart showing control for calculating a DOWN burnouttemperature.

FIG. 18 is a flowchart showing control for calculating a predictedtemperature during a normal DOWN shift.

FIG. 19 is a flowchart showing control for calculating a predictedtemperature during a second synchronized shift.

FIG. 20 is a time chart of an UP shift.

FIG. 21 is a time chart of a DOWN shift.

FIG. 22 is a time chart showing actions of the shift control device foran automatic transmission according to this embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of this invention will be described in detail below withreference to the figures and so on. FIG. 1 is a functional block diagramshowing the constitution of a shift control device for an automatictransmission according to this embodiment. FIG. 2 is a skeleton diagramshowing the constitution of the automatic transmission. As shown in FIG.1, the shift control device comprises a controller 1, various sensorsincluding an input shaft rotation speed sensor (turbine shaft rotationspeed sensor) 12 that detects a rotation speed NT of a turbine 25 and aturbine shaft 10, an output shaft rotation speed sensor (vehicle speedsensor) 13 that detects a rotation speed No of an output shaft 28, anoil temperature sensor 14 that detects a temperature of ATF (automatictransmission oil), a throttle sensor 30 that detects a throttle openingof an engine, not shown in the figures, an air flow sensor 31 thatdetects an intake air amount of the engine, and an engine rotation speedsensor 32 that detects an engine rotation speed NE, and a hydrauliccircuit 11 of an automatic transmission 7. Using the controller 1, theshift control device performs shift control to determine a desiredtarget gear position on the basis of detection signals from theaforementioned sensors 12, 13, 14, 30, 31, 32, etc., and to achieve thetarget gear position via the hydraulic circuit 11.

The gear position of the automatic transmission 7 is determinedaccording to engagement relationships among a planetary gear unit and aplurality of frictional elements, including hydraulic clutches,hydraulic brakes, and so on, provided in the automatic transmission 7.For example, FIG. 1 shows a case in which the automatic transmission 7has four gear positions, and therefore a first clutch 15, a secondclutch 17, a third clutch 19, a first brake 22, and a second brake 23are provided as the frictional elements. The automatic transmission 7 isshown in detail in FIG. 2. In FIG. 2, reference numerals denoting therespective frictional elements correspond to those in FIG. 1.

The frictional elements 15, 17, 19, 22, 23 are controlled by thecontroller 1 via the hydraulic circuit 11 shown in FIG. 1. Morespecifically, a plurality of solenoid valves, not shown in the figures,are provided in the hydraulic circuit 11, and by driving(duty-controlling) these solenoid valves appropriately, ATF deliveredfrom an oil pump is supplied to the frictional elements 15, 17, 19, 22,23. The controller 1 determines a target gear position on the basis ofthe throttle opening detected by the throttle sensor 30 and the vehiclespeed calculated on the basis of the rotation speed No of the outputshaft 28 detected by the output shaft rotation speed sensor 13. Thecontroller then outputs a drive signal (duty ratio signal) to thesolenoid valves of the frictional elements 15, 17, 19, 22, 23 thatcontribute to a shift to the determined target gear position. It shouldbe noted that the pressure of the ATF is regulated to a predeterminedoil pressure (line pressure) by a regulator valve not shown in thefigures, and ATF regulated to this line pressure is supplied to thehydraulic circuit 11 to activate the respective frictional elements 15,17, 19, 22, 23.

A shift map 3 is provided in the controller 1. Further, a switch lever(not shown) for switching an operating mode is attached to the automatictransmission 7, and a driver can manually select a shift range, such asa parking range, a traveling range (for example, a first speed to afourth speed), a neutral range, and a reverse range, by operating theswitch lever.

The traveling range includes two shift modes, namely an automatic shiftmode and a manual shift mode. When the automatic shift mode is selected,a shift determination is performed in accordance with the shift map 3,which is set in advance on the basis of a throttle opening θ_(TH) and avehicle speed V, and a shift is implemented automatically in accordancewith this determination. When the manual shift mode is selected, on theother hand, the gear position is shifted to a gear position selected bythe driver, regardless of the shift map 3, and fixed thereafter.

Characteristics such as those shown in FIG. 4, for example, are recordedin the shift map 3. During a normal shift in which the shift isimplemented automatically, a target gear position corresponding to thevehicle speed V detected by the vehicle speed sensor 13 and the throttleopening θ_(TH) detected by the throttle sensor 30 is set on the basis ofthe shift map 3 shown in FIG. 4. The frictional elements, including thefirst to third clutches 15, 17, 19 and the first and second brakes 22,23 described above, are controlled by the solenoid valves setrespectively therein, and each gear position is establishedautomatically through engagement and disengagement combinations such asthose shown in FIG. 3. In FIG. 3, the circle marks indicate that thecorresponding clutch or brake is engaged.

As shown in FIG. 3, when the first clutch 15 and second brake 23 areengaged and the second clutch 17, third clutch 19, and first brake 22are disengaged, for example, a second speed is reached. A shift from thesecond speed to a third speed is achieved by disengaging the engagedsecond brake 23 and engaging the second clutch 17. The engagement stateof the frictional elements 15, 17, 19, 22, 23 is controlled by thecontroller 1, and the gear position is determined according to theengagement relationships among the frictional elements 15, 17, 19, 22,23. Moreover, shift control is performed while measuring the engagementand disengagement timing appropriately.

During a shift, a drive signal is output to each solenoid valve from thecontroller 1, and on the basis of the drive signal, the solenoid valveis driven by a predetermined duty value (duty ratio). As a result,optimum shift control is executed so as to provide a favorable shiftfeeling.

Next, the main parts of this embodiment will be described in detail. Thedevice constantly calculates a current thermal load state (temperature)of each frictional element (to be referred to simply as “clutch”hereafter). When a shift is determined, a temperature increase T_(INH)of the corresponding clutch during the shift is estimated, and on thebasis of this result, the shift is either prohibited or permitted.

More specifically, when an operating point crosses an upshift line and adownshift line of the shift map 3 continuously and repeatedly, a 3-4shift and a 4-3 shift may be performed repeatedly between the thirdspeed and fourth speed, for example, leading to a continuous 3-4-3-4- .. . shift. A 3-4-3-4- . . . continuous shift may be performed similarlywhen the driver switches the shift lever frequently between the thirdspeed and fourth speed.

When a continuous shift is performed in this manner, a specific clutch(in the case of a 3-4 continuous shift, the first clutch 15 and secondbrake 23; see FIG. 3) is repeatedly engaged and disengaged. Whenengagement and disengagement are executed repeatedly over a short timeperiod in this manner, the thermal capacity of the clutch increases (thetemperature rises), and as a result, burns may occur on the clutch orbrake.

Further, when the thermal load state of the clutch is predicted andshifts are prohibited using a timer alone, without taking the type ofshift, the engagement/disengagement state, and input torque intoconsideration, as in the prior art, the precise temperature of theclutch and so on cannot be obtained. Therefore, a threshold fordetermining that shifts are to be prohibited is set at a value having asufficient margin to ensure that the clutch does not reach a burnouttemperature even when a shift that generates a maximum amount of heat isperformed. Accordingly, shifts may be prohibited even in a state where ashift may be permitted, and as a result, drivability is impaired.

In this embodiment, the thermal load state (current temperature) of eachclutch is calculated, and when a shift is determined, the increase inthe temperature of the clutch is predicted so that the determination asto whether to prohibit or permit the shift can be made accurately. Morespecifically, as shown in FIG. 5, the controller 1 includes, in additionto the shift map 3, current temperature calculating means 101 forcalculating the current temperature of each clutch, predictedtemperature increase calculating means 102 for predicting thetemperature increase T_(INH) of the clutch during the next shift,predicted temperature calculating means 103 for determining a predictedtemperature T_(ES) of the clutch following the next shift on the basisof the current temperature and predicted temperature increase of theclutch, comparing means 109 for comparing the predicted temperatureT_(ES) to a predetermined threshold, and shift prohibiting/switchingmeans 104 for permitting or prohibiting the next shift or switching toanother shift on the basis of whether or not the predicted temperatureT_(ES) is determined to be equal to or greater than the predeterminedvalue by the comparing means 109.

First, the current temperature calculating means 101 will be described.

The current temperature calculating means 101 successively calculatesand updates the current temperature of each clutch, an initial value ofwhich is set at an ATF temperature T_(OIL), obtained from the oiltemperature sensor 14, at the time of engine startup. The reason forsetting the initial value in this manner is that at the time of enginestartup, the temperature of the respective clutches of the transmission7 may be considered substantially equal to the oil temperature T_(OIL).

FIG. 6 is a diagram verifying the appropriateness of applying the oiltemperature T_(OIL) as the initial value of the clutch temperature atthe time of engine startup. In the figure, V_(SP) denotes the vehiclespeed.

As shown in the figure, the temperature of the clutch (corresponding tothe second brake 23 in this embodiment; see FIG. 3) that is engaged whenshifting from the first speed to the second speed is intentionally heldat a temperature (burnout temperature) at which burns may occur, and inthis state, the vehicle speed is reduced along a fixed gradient. Then,when the vehicle speed V_(SP)=0 following a downshift to the firstspeed, the engine is halted by switching an ignition OFF (IGN-OFF) (seet1 in the figure). Following IGN-OFF, the engine is restarted (IGN-ON)(see t2) and an accelerator is fully opened to perform an upshift to thesecond speed (see t3).

Here, a case in which approximately ten seconds are required between thedownshift to the first speed (see t0) and the upshift to the secondspeed (see t3) was simulated, and it was confirmed that since the clutchtemperature decreases gradually along a predetermined gradient from t0,approximately ten seconds are sufficient for the clutch temperature todecrease reliably to approximately the oil temperature T_(OIL) in an oilpan.

Hence, it was confirmed experientially that even when the engine isrestarted immediately after being stopped, the clutch temperatureapproximately reaches the oil temperature T_(OIL), and therefore the oiltemperature T_(OIL) may be set as the initial temperature at the time ofengine startup.

After setting the initial value of the clutch temperature in the mannerdescribed above, the current temperature calculating means 101calculates a clutch temperature Tc using different methods according tothe current state of the clutch. More specifically, the thermal load(heat generation amount T_(up)) of the clutch differs between an engagedperiod and a disengaged period and also between a shift transitionperiod and a steady state period. The thermal load of the clutch alsodiffers between an upshift and a downshift. Therefore, as shown in FIG.5, the current temperature calculating means 101 includes heatgeneration amount calculating means 105 for calculating heat generationduring a transition between engagement and disengagement of the clutch,and heat radiation amount calculating means 106 for calculating heatradiation during engagement and disengagement steady states. Further,the heat generation amount calculating means 105 is provided withengagement transition heat generation amount calculating means 107 forcalculating heat generation during an engagement transition, anddisengagement transition heat generation amount calculating means 108for calculating the heat radiation amount during a disengagementtransition.

It should be noted that in this embodiment, an “engagement transition”indicates that a clutch to be engaged is in a torque phase or an inertiaphase, while a “disengagement transition” indicates that a clutch to bedisengaged is in the torque phase or the inertia phase. Further, an“engagement steady state” indicates that the subject clutch is fullyengaged and in neither the torque phase nor the inertia phase,irrespective of whether a shift command has been issued or a shift isnot underway. Further, a “disengagement steady state” indicates that thesubject clutch is fully disengaged.

FIG. 7 is a view showing a characteristic of actual temperaturevariation accompanying engagement and disengagement of the clutch duringan upshift. As shown in the figure, the greatest temperature increaseoccurs between the start of clutch engagement and the end of clutchengagement. Also, the temperature variation gradient is greatest at thistime. Once the clutch is engaged and enters a steady state, thetemperature decreases along a fixed gradient. When disengagement of theclutch begins, the temperature reduction up to this point and atemperature increase caused by frictional heat that is generated byrelative rotation of the clutch cancel each other out such that thetemperature becomes substantially constant and variation in the clutchtemperature is minute (shown in FIG. 7 as a fixed clutch temperatureTc).

Once disengagement of the clutch is complete (during a disengagementsteady state), the temperature falls along a predetermined gradient. Atthis time, the temperature decrease gradient following clutchdisengagement (during the disengagement steady state) is larger (theincline is larger) than the temperature decrease gradient followingclutch engagement (during the engagement steady state).

Hence, the current temperature calculating means 101 calculates theclutch temperature Tc taking into account this temperature variationcharacteristic. Calculation of the clutch temperature Tc by the currenttemperature calculating means 101 will now be described specifically. Onthe basis of information from the shift map 3, the current temperaturecalculating means 101 inputs the current gear position, as well as thetarget gear position when a shift is determined. The turbine rotationspeed NT and engine rotation speed NE are also input into the currenttemperature calculating means 101 from the turbine rotation speed sensor12 and the engine rotation speed sensor 32.

Of the plurality of clutches, the clutches that are in the engagementsteady state or the disengagement steady state do not come into slidingcontact with each other while still having capacity as a result of beingin a steady state, and therefore frictional heat is not generated in theclutches and the temperature thereof does not rise. Hence, the heatradiation amount is calculated by the heat radiation amount calculatingmeans 106. Here, when a shift operation by the transmission 7 is notunderway, the clutches in the engagement steady state or thedisengagement steady state correspond to all of the clutches, and when ashift operation is underway, the clutches in the engagement steady stateor the disengagement steady state correspond to the clutches that do notcontribute to the shift operation, for example the third clutch 19 andfirst brake 22 during a shift from the second speed to the third speed.

The heat radiation amount calculating means 106 calculates a heatradiation amount (temperature reduction margin) T_(down) on the basis ofthe following Equations (1) and (2). It should be noted that in thecontrol of the controller 1, the heat generation amount T_(up) istreated as + and the heat radiation amount is treated as −, andtherefore, in the following Equations (1) and (2), the heat radiationamount T_(down)<0.

Engaged state:T _(down) =−A×t _(c)(t≦t1), T _(down) =−B×t _(c)(t1≦t)  (1)

In Equation (1), A is a variable, B is a constant, t_(c) is an interval,t is an elapsed time following shift completion, and t1 is apredetermined time period.

Disengaged state:T _(down) =−C×t _(c)(t≦t1), T _(down) =−D×t _(c)(t1≦t)  (2)

In Equation (2), C is a variable, D is a constant, t_(c) is theinterval, t is the elapsed time following shift completion, and t1 isthe predetermined time period.

More specifically, the heat radiation amount calculating means 106calculates the heat radiation amount T_(down) assuming that between theachievement of a steady state following completion of the shift and theelapse of the predetermined time period t1, the clutch temperature Tcdecreases along gradients A and C, which are variables, and calculatesthe heat radiation amount T_(down) assuming that after the elapse of thepredetermined time period t1 from completion of the shift, the clutchtemperature Tc decreases along gradients B and D, which are constants.The variables A and C are values determined on the basis of atemperature difference between the current clutch temperature Tc and theoil temperature T_(OIL), and are set such that the gradient increases asthe temperature difference increases. Further, the constant gradients B,C are set such that B>C, and such that the temperature decreases along asharper gradient in the disengagement steady state, as shown in FIG. 7.The reason for this is that in the disengagement steady state, it iseasier to supply lubricating oil to a facing surface of the clutch thanin the engagement steady state, and as a result, greater heat radiationcan be performed.

By adding the currently calculated heat radiation amount T_(down) to thepreviously calculated current clutch temperature Tc, the new currentclutch temperature Tc is calculated.

When the clutch is in the engagement steady state or the disengagementsteady state, the clutch temperature Tc falls along a predeterminedgradient, as shown in Equations (1) and (2), and therefore, when thecalculation subject clutch remains in a steady state for a long time, atemperature that is not possible in reality (for example, a lowertemperature than the oil temperature T_(OIL)) is calculated erroneously.

Therefore, the heat radiation amount calculating means 106 is providedwith a function for resetting calculation of the heat radiation amountT_(down) using the Equations (1) and (2) (or clipping a lower limitvalue thereof) when the clutch remains in the engagement steady state orthe disengagement steady state for a predetermined time period. In otherwords, a reset determination timer, not shown in the figures, isprovided in the heat radiation amount calculating means 106, and when itis determined that the engagement steady state or disengagement steadystate has begun, the timer starts to count.

When the clutch is in the engagement steady state or the disengagementsteady state and the timer count indicates that this state has remainedunchanged for a predetermined time period, calculation of the clutchtemperature Tc based on the Equations (1) and (2) is canceled.Furthermore, in this case, the clutch temperature Tc should havedecreased sufficiently to be equal to the oil temperature T_(OIL), andtherefore the clutch temperature Tc is matched to the current oiltemperature T_(OIL) thereafter.

Further, if the current clutch temperature Tc falls to or below the oiltemperature T_(OIL) even though the timer count has not exceeded thepredetermined time period, clutch temperature Tc=oil temperature T_(OIL)is set thereafter.

On the other hand, if the state of the clutch shifts to an engagementtransition or a disengagement transition within the predetermined timeperiod following the start of the timer count, the timer is reset suchthat the count returns to its initial value. Then, when the clutchreturns to a steady state from the transition state, counting is startedfrom the initial value.

Here, using FIG. 8, an action of the reset determination timer when acontinuous shift is performed between an N^(th) speed and an N+1^(th)speed will be described, FIG. 8A is a view illustrating variation in theclutch temperature Tc, and FIG. 8B is a view illustrating the count ofthe reset determination timer.

As shown in FIG. 8A, when a continuous shift occurs, the clutchtemperature Tc rises every time the clutch is engaged. It should benoted that the clutch temperature Tc decreases when the clutch is in theengagement steady state or the disengagement steady state, but when acontinuous shift is performed over a short time period, the temperaturedecrease is smaller than the temperature increase during the clutchengagement transition.

Meanwhile, as shown in FIG. 8B, the timer count is reset every time ashift is begun (during a transition). In this example, the timer countcontinues when the clutch shifts to the engagement steady state. Whenthe timer count reaches a predetermined value, as shown in FIG. 8A, itis determined thereafter that the clutch temperature Tc has fallen tothe oil temperature T_(OIL), and therefore the clutch temperature Tc isset at the oil pan temperature T_(OIL). Further, the timer count is heldat a set value or a maximum value set at a larger value than the setvalue.

Next, calculation of the temperature (generated heat) of the clutchduring an engagement transition or a disengagement transition will bedescribed.

In this case, the current temperature of the clutch is calculatedperiodically by the heat generation amount calculating means 105. First,when the clutch is determined to be in a transition state on the basisof information from the turbine rotation speed sensor 12 and so on, theheat generation amount calculating means 105 determines whether theclutch is in an engagement transition state or a disengagementtransition state.

When it is determined that the clutch is in an engagement transitionstate (for example, the second clutch 17 during a 2→3 shift), theengagement transition heat generation amount calculating means 107provided in the heat generation amount calculating means 105 calculatesthe heat generation amount T_(up) of the clutch.

On the basis of information from the shift map 3, the engagementtransition heat generation amount calculating means 107 determineswhether the current shift is an upshift or a downshift. When the clutchis in an engagement transition state, the heat generation amount differsgreatly between an upshift and a downshift. More specifically, the heatgeneration amount when the clutch is in an engagement transition stateduring an upshift is greater than that of a downshift. When the clutchis in an engagement transition state during a downshift, on the otherhand, the heat generation amount is smaller than that of an upshift.

The reason for this is that when a disengagement side clutch isdisengaged during a downshift, the engine rotation speed increases underits own power such that an engagement side clutch is engaged at asynchronized timing, and therefore the heat generation amount T_(up) inthe engagement side clutch is smaller than the heat generation amountT_(up) thereof during an upshift.

Hence, in this embodiment, when an upshift is determined during anengagement transition state, the heat generation amount T_(up) of theclutch is calculated on the basis of the following Equation (3), andwhen a downshift is determined, the heat generation amount T_(up) iscalculated on the basis of the following Equation (4).T _(up)=(ΔN×T _(in) ×Δt/1000)×A×α  (3)T _(up)=0  (4)

In Equation (3), ΔN is a relative rotation speed of the clutch, T_(in)is a transmission torque of the clutch, Δt is a very short shift period,A is a constant for converting an energy amount into a temperature, andα is a matching constant (correction coefficient). The relative rotationspeed ΔN of the clutch is calculated on the basis of the turbinerotation speed NT obtained by the turbine rotation speed sensor 12, theoutput shaft rotation speed No obtained by the output shaft rotationspeed sensor 13, and a gear ratio of the respective gears of thetransmission. Further, the transmission torque of the clutch iscalculated from the duty values of the solenoid valves provided inrelation to the respective clutches, or in other words oil pressurevalues.

Further, even during an engagement transition, the heat generationamount T_(up) during a downshift is slight, and therefore, in thisembodiment, the heat generation amount T_(UP) during a downshift is setat 0, as shown in Equation (4). The reason for this is that when theclutch enters an engagement transition state, the temperature reduction(heat radiation) generated by the lubricating oil and the comparativelysmall temperature increase generated by heat generation cancel eachother out, as noted above, and therefore the temperature remainssubstantially constant.

Hence, during an upshift, the current clutch temperature Tc iscalculated by calculating the heat generation amount T_(UP) periodicallywhile the shift is underway, and adding the clutch temperature Tccalculated in the previous control period to the calculated heatgeneration amount T_(UP). As described above, the initial value of theclutch temperature Tc is set at the ATF temperature T_(OIL) obtained bythe oil temperature sensor 14.

When the clutch is determined to be in a disengagement transition state,on the other hand (for example, the second brake 23 during a 2→3 shift),the disengagement transition heat generation amount calculating means108 provided in the heat generation amount calculating means 105calculates the heat generation amount T_(up) of the clutch.

On the basis of information from the shift map 3, the disengagementtransition heat generation amount calculating means 108 determineswhether the current shift is an upshift or a downshift. When the clutchis in a disengagement transition state, the heat generation amountdiffers greatly between an upshift and a downshift. In contrast to anengagement transition, the heat generation amount during a downshiftengagement transition is greater than that of an upshift. When theclutch is in a disengagement transition state during an upshift, on theother hand, the heat generation amount is smaller than that of adownshift.

Hence, when an upshift is determined, the heat generation amount T_(up)is calculated on the basis of the above Equation (4), and when adownshift is determined, the heat generation amount T_(up) is calculatedon the basis of the Equation (3).

When a shift is determined by the controller 1 while calculating thecurrent clutch temperature Tc in the manner described above, thetemperature increase T_(INH) of the clutch that contributes to the shiftupon execution of the next shift from the current temperature state ispredicted.

The temperature increase T_(INH) is predicted by the predictedtemperature increase calculating means 102 provided in the controller 1.Here, as shown in FIG. 5, the predicted temperature increase calculatingmeans 102 comprises UP shift predicted temperature increase calculatingmeans 111 for predicting the temperature increase T_(INH) of the clutchduring an upshift, normal DOWN shift predicted temperature increasecalculating means 112 for predicting the temperature increase T_(INH) ofthe clutch during a normal downshift, PYDOWN shift predicted temperatureincrease calculating means 113 for predicting the temperature increaseT_(INH) of the clutch during a PYDOWN shift to be described below, andsecond synchronized shift predicted temperature increase calculatingmeans 114 for predicting the temperature increase T_(INH) of the clutchduring a second synchronized shift.

When an upshift or a downshift is determined by the controller 1, thetemperature increase T_(INH) is predicted before an upshift command or adownshift command is actually issued. Calculation methods employed bythe respective predicted temperature increase calculating means will bedescribed below.

As shown in FIG. 5, when the predicted temperature increase T_(INH)during the next shift is calculated by the predicted temperatureincrease calculating means 102 in this manner, the predicted temperatureincrease T_(INH) and the current clutch temperature Tc calculated by thecurrent temperature calculating means 101 are input into the predictedtemperature calculating means 103.

The predicted temperature increase T_(INH) is added to the currentclutch temperature Tc by the predicted temperature calculating means103, whereby a predicted temperature T_(ES) upon completion of the nextshift is calculated.

Further, as shown in FIG. 5, the controller 1 is provided with thresholdstoring means 110. The threshold storing means 110 stores an UP burnouttemperature and a DOWN burnout temperature. The UP burnout temperatureis a temperature at which the clutch burns out when the clutchtemperature Tc exceeds, and is used during an upshift (also written asUP shift hereafter) to determine whether or not the post-shift clutchtemperature Tc exceeds the UP burnout temperature. The DOWN burnouttemperature is a lower temperature than the UP burnout temperature, andis used during a downshift (also written as DOWN shift hereafter) todetermine whether or not the post-shift clutch temperature Tc exceedsthe DOWN shift temperature, and is obtained by subtracting a temperatureincrease corresponding to the maximum heat generation amount T_(up)generated by a PYUP shift from the UP burnout temperature. A PYUP shiftmeans a shift such that a determined shift is executed in a shift modethat generates a smaller heat generation amount T_(up) than a normal UPshift, and will be described below.

In the comparing means 109, the predicted temperature T_(ES) is comparedto the UP burnout temperature or the DOWN burnout temperature, and whenit is determined that the predicted temperature T_(ES) is equal to orgreater than the UP burnout temperature or the DOWN burnout temperature,the determined upshift or downshift is either prohibited or switched toanother shift by the shift prohibiting/switching means 104. Here,another shift indicates a PYUP shift relating to an upshift performed ina normal shift mode or a PYDOWN shift relating to a downshift performedin the normal shift mode. When the predicted temperature T_(ES) isdetermined to be lower than the UP burnout temperature or the DOWNburnout temperature, on the other hand, the determined shift ispermitted, and either an upshift or a downshift is executed in thenormal shift mode.

Further, as shown in FIG. 5, the controller 1 is provided withcontinuous change-mind shift allowable number calculating means 115. Achange-mind indicates that a shift to the n^(th) speed is determinedanew during an operation to shift from the n^(th) speed to the n+1^(th)speed or an n−1^(th) speed. When the shift determination is determinedto be a change-mind, a number of allowable continuous change-mind shiftsis calculated on the basis of the current clutch temperature Tc, withoutpredicting the temperature increase T_(INH) of the clutch.

A current number of continuous change-mind shifts is then compared tothe allowable number of continuous change-mind shifts in the comparingmeans 109. When it is determined that the current number of continuouschange-mind shifts is equal to or greater than the allowable number ofcontinuous change-mind shifts, execution of the determined upshift ordownshift is prohibited. When it is determined that the current numberof continuous change-mind shifts is smaller than the allowable number ofcontinuous change-mind shifts, on the other hand, execution of thedetermined upshift or downshift is permitted.

By means of the control described above, when there is a danger of burnsoccurring on a clutch, the next upshift or downshift is prohibited orthe shift mode is switched from the normal shift mode to another shiftmode, and when it is determined that there is no danger of burnsoccurring on the clutch, the upshift or downshift is permitted. As aresult, shifts can be prohibited or permitted appropriately, inaccordance with the thermal load state of the clutch.

The PYUP shift and PYDOWN shift will now be described. The PYUP shiftand PYDOWN shift are shift modes in which the input torque is identicalto the shift mode of a normal upshift and a normal downshift, but theshift time is shorter, leading to a corresponding decrease in the heatgeneration amount T_(up). Specifically, the shift time is shortened byincreasing the increase gradient and decrease gradient of the oilpressure.

It should be noted that hereafter in this specification, the term“upshift” will be used to signify a switch to a High side gear position.The term “UP shift” indicates an upshift performed in the normal shiftmode, and will be used mainly to clarify differences with an upshiftperformed in another shift mode (for example, a PYUP shift). Similarly,the term “downshift” will be used to signify a switch to a Low side gearposition. The term “DOWN shift” indicates a downshift performed in thenormal shift mode, and will be used mainly to clarify differences with adownshift performed in another shift mode (for example, a PYDOWN shift).

First, a PYUP shift will be described with reference to FIG. 9. FIG. 9is a time chart showing a gear ratio, an oil pressure command value of adisengagement side clutch, an oil pressure command value of anengagement side clutch and engine torque variation during a PYUP shift,in which the broken lines indicate the normal shift mode (a normal UPshift) and the solid lines indicate the low heat generation amount shiftmode (a PYUP shift).

As shown by the solid lies in FIG. 9, the engagement side clutch iscontrolled such that the oil pressure increase gradient during thetorque phase (t1 to t2) and the oil pressure increase gradient duringthe inertia phase (t2 to t3) are larger than those of the normal shiftmode (a normal UP shift). Further, the disengagement side clutch iscontrolled to increase the oil pressure decrease gradient during thetorque phase (t1 to t2). The reason for this is that if thedisengagement side clutch still has capacity when the engagement sideclutch begins to take on capacity, interlocking may occur.

Thus, whereas a time period of (t4-t1) is required for the gear ratio toshift from the n^(th) speed to the n+1^(th) speed in the normal shiftmode (a normal UP shift), a PYUP shift requires only a time period of(t3-t1), and therefore the shift can be shortened by a time period of(t4-t3). As a result, the heat generation amount T_(up) of theengagement side clutch decreases in accordance with the shortened timeperiod.

Engine torque reduction control is performed during the inertia phase ofan upshift, but during a PYUP shift, the torque reduction amount is setto be larger, and therefore, even though the engagement side clutch isengaged in a short time period during a PYUP shift, an increase in shiftshock can be suppressed.

A PYDOWN shift will now be described similarly with reference to FIG.10. FIG. 10 is a time chart showing the gear ratio and variation in theoil pressure command value of the disengagement side clutch and the oilpressure command value of the engagement side clutch during a PYDOWNshift. In FIG. 10, the broken lines indicate the normal shift mode (anormal DOWN shift) and the solid lines indicate the low heat generationamount shift mode (a PYDOWN shift).

As shown by the solid lies in FIG. 10, the disengagement side clutch iscontrolled such that the oil pressure decrease gradient from the startof the shift to the start of the inertia phase (t1 to t2) and the oilpressure increase gradient during the inertia phase (t2 to t3) arelarger than those of a normal shift. Further, the engagement side clutchis controlled to increase the oil pressure increase gradient during theinertia phase (t2 to t3).

Thus, whereas a time period of (t6-t1) is required for the gear ratio toshift from the n^(th) speed to the n−1^(th) speed during a normal shift,a PYDOWN shift requires only a time period of (t4-t1), and therefore theshift can be shortened by a time period of (t6-t4). As a result, theheat generation amount T_(up) of the disengagement side clutch decreasesin accordance with the shortened time period.

The control performed by the controller 1, described above withreference to FIG. 5, will be described in detail below using flowchartsin FIGS. 11 to 18. It should be noted that the flowcharts shown in FIGS.11 to 18 are executed on each clutch.

First, referring to FIG. 11, the control content of the currenttemperature calculating means 101 will be described.

In a step S1, information such as the current engine rotation speed NE,turbine rotation speed NT, oil temperature T_(OIL), and vehicle speed Nois obtained.

In a step S2, a determination is made as to whether the clutch is in theengagement steady state, the disengagement transition state, thedisengagement steady state, or the engagement transition state.

When the clutch is in the engagement steady state, the routine advancesto a step S3, where the reset determination timer is counted up. Theroutine then advances to a step S4, where an engagement period heatradiation amount T_(down) is calculated. Calculation of the engagementperiod heat radiation amount T_(down) will be described below.

When the clutch is in the disengagement transition state, the routineadvances to a step S5, where a determination is made as to whether theshift type is an upshift or a downshift. When the shift type is adownshift, the routine advances to a step S6, where the resetdetermination timer is cleared. The routine then advances to a step S7,where a disengagement period heat generation amount T_(up) iscalculated. The disengagement period heat generation amount T_(up) iscalculated on the basis of Equation (3). When the shift type is anupshift, the routine advances to a step S8, where the resetdetermination timer is cleared. The routine then advances to a step S9,where the heat generation amount T_(up) is set at 0 on the basis ofEquation (4).

When the clutch is in the disengagement steady state, the routineadvances to a step S10, where the reset determination timer is countedup. The routine then advances to a step S11, where a disengagementperiod heat radiation amount T_(down) is calculated. Calculation of thedisengagement period heat radiation amount T_(down) will be describedbelow.

When the clutch is in the engagement transition state, the routineadvances to a step S12, where a determination is made as to whether theshift type is an upshift or a downshift. When the shift type is adownshift, the routine advances to the step S8, where the resetdetermination timer is cleared. The routine then advances to the stepS9, where the heat generation amount T_(up) is set at 0 on the basis ofEquation (4). When the shift type is an upshift, the routine advances toa step S13, where the reset determination timer is cleared. The routinethen advances to a step S14, where the engagement period heat generationamount T_(up) is calculated on the basis of Equation (3).

In a step S15, a determination is made as to whether or not the resetdetermination timer is equal to or greater than a clutch resetting settime period. When the reset determination timer is equal to or greaterthan the clutch resetting set time period, the routine advances to astep S16, where the current clutch temperature Tc is set at the oiltemperature T_(OIL). The processing is then terminated.

When the reset determination timer is smaller than the clutch resettingset time period, the routine advances to a step S17, where either theheat generation amount T_(up) or the heat radiation amount T_(down) isadded to the current clutch temperature Tc. It should be noted that theheat radiation amount T_(down) is a negative value. Here, the clutchresetting set time period is a time period long enough to determine thatthe clutch temperature Tc has decreased sufficiently to be equal to theoil temperature T_(OIL) after the clutch has remained in the engagementsteady state or disengagement steady state continuously for apredetermined time period.

In a step S18, a determination is made as to whether or not the currentclutch temperature Tc is equal to or lower than the oil temperatureT_(OIL). When the current clutch temperature Tc is equal to or lowerthan the oil temperature T_(OIL), the routine advances to the step S16,where the current clutch temperature Tc is set at the oil temperatureT_(OIL). When the current clutch temperature Tc is higher than the oiltemperature T_(OIL), the processing is terminated. In other words, inreality the clutch temperature Tc is unlikely to fall below the oiltemperature T_(OIL), and therefore, when the calculated clutchtemperature Tc is lower than the oil temperature T_(OIL), the clutchtemperature Tc is set at the oil temperature T_(OIL).

Calculation of the engagement period heat radiation amount T_(down) inthe step S4 of FIG. 11 will now be described with reference to theflowchart of FIG. 12. It should be noted that the disengagement periodheat radiation amount T_(down) of the step S11 is calculated using asimilar method to calculation of the engagement period heat radiationamount T_(down) to be described below.

In a step S101, a determination is made as to whether or not a shift hasjust been completed. When a shift has just been completed, the routineadvances to a step S102, and when a shift has not just been completed,the routine advances to a step S103.

In the step S102, the temperature decrease gradient is set on the basisof the temperature difference between the current clutch temperature Tcand the oil temperature T_(OIL). The temperature decrease gradientcorresponds to A and C in Equations (1) and (2), and is set to increaseas the temperature difference between the current clutch temperature Tcand the oil temperature T_(OIL) increases.

In the step S103, the timer is counted.

In a step S104, a determination is made as to whether or not the timeris at or above a predetermined value. When the timer is at or above thepredetermined value, the routine advances to a step S105, where thetemperature decrease gradient is set at a predetermined gradient (afixed value).

In a step S106, the current engagement period heat radiation amountT_(down) is calculated from the time elapsed since the start of theshift (the value of the timer) and the temperature decrease gradient,whereupon the processing is terminated. Here, the predetermined valuecorresponds to t1 in Equations (1) and (2), and indicates the timerequired for the temperature decrease gradient to become substantiallyconstant, regardless of the temperature at the start of heat radiation.The predetermined value is set at 5 seconds, for example.

Next, referring to FIGS. 13 and 14, the control content of the predictedtemperature increase calculating means 102, the predicted temperaturecalculating means 103, the threshold calculating means 110, thecontinuous change-mind shift allowable number calculating means 115, thecomparing means 109, and the shift prohibiting/switching means 104 willbe described.

In a step S21, a determination is made as to whether or not a shift hasbeen determined. When a shift has been determined, the routine advancesto a step S22, and when a shift has not been determined, the processingis terminated.

In the step S22, a determination is made as to whether or not thedetermined shift is a change-mind shift. When the determined shift is achange-mind shift, the routine advances to a step S50, and when thedetermined shift is not a change-mind shift, the routine advances to astep S23. A change-mind shift indicates that a shift to the n^(th) speedis determined anew during an operation to shift from the n^(th) speed tothe n+1^(th) speed or the n−1^(th) speed.

In the step S23, a determination is made as to whether or not the shiftis an upshift or a downshift. When the shift is an upshift, the routineadvances to a step S24, and when the shift is a downshift, the routineadvances to a step S29.

In the step S24, an UP shift predicted temperature increase iscalculated. The UP shift predicted temperature increase is the predictedtemperature increase T_(INH) of the clutch to be engaged during theupshift, and a calculation method thereof will be described in detailbelow.

In a step S25, an UP shift predicted temperature T_(ES) is determined byadding the UP shift predicted temperature increase to the current clutchtemperature Tc.

In a step S26, a determination is made as to whether or not the UP shiftpredicted temperature T_(ES) is equal to or greater than the UP burnouttemperature, or in other words whether or not the UP shift predictedtemperature T_(ES) is within a temperature region at or above the UPburnout temperature. When the UP shift predicted temperature T_(ES) islower than the UP burnout temperature, the routine advances to a stepS27, where an UP shift is performed in the normal shift mode. When theUP shift predicted temperature T_(ES) is equal to or greater than the UPburnout temperature, the routine advances to a step S28, where a PYUPshift is performed in the low heat generation amount shift mode. Here, anormal UP shift performed in the normal shift mode is executed bysetting the oil pressure such that the driver does not feel shift shock.In a PYUP shift, the increase rate of the oil pressure supplied to theclutch is increased beyond that of a normal UP shift to shorten the timerequired to engage the clutch. It should be noted that during a PYUPshift, the torque reduction amount of the engine is increased beyondthat of a normal UP shift. In so doing, an increase in shift shock canbe suppressed, and since the input torque decreases, the heat generationamount T_(up) also decreases.

When the shift is determined to be a downshift in the step S23, on theother hand, the routine advances to a step S29, where the DOWN burnouttemperature is calculated. A calculation method for calculating the DOWNburnout temperature will be described in detail below.

In the step S30, a determination is made as to whether or not thedownshift has been caused by depressing the accelerator. If so, theroutine advances to a step S40, and if not, the routine advances to astep S31.

In the step S31, a normal DOWN shift predicted temperature increase iscalculated. The normal DOWN shift predicted temperature increase is thepredicted temperature increase T_(INH) of the clutch that is disengagedduring a normal downshift, and a calculation method thereof will bedescribed in detail below.

In a step S32, a normal DOWN shift predicted temperature T_(ES) isdetermined by adding the normal DOWN shift predicted temperatureincrease to the current clutch temperature Tc.

In a step S33, a determination is made as to whether or not the normalDOWN shift predicted temperature T_(ES) is equal to or greater than theDOWN burnout temperature, or in other words whether or not the normalDOWN shift predicted temperature T_(ES) is within a temperature regionat or above the DOWN burnout temperature. When the normal DOWN shiftpredicted temperature T_(ES) is lower than the DOWN burnout temperature,the routine advances to a step S34, where a normal DOWN shift isperformed. When the normal DOWN shift predicted temperature T_(ES) isequal to or greater than the DOWN burnout temperature, the routineadvances to a step S35.

In the step S35, a PYDOWN shift predicted temperature increase iscalculated. The PYDOWN shift predicted temperature increase is thepredicted temperature increase T_(INH) of the clutch that is disengagedduring a PYDOWN shift, and a calculation method thereof will bedescribed in detail below. A PYDOWN shift is a shift in which the timerequired to disengage the clutch is shortened by increasing the decreaserate of the oil pressure supplied to the clutch beyond that of a normalDOWN shift performed in the normal shift mode.

In a step S36, a PYDOWN shift predicted temperature T_(ES) is determinedby adding the PYDOWN shift predicted temperature increase T_(INH) to thecurrent clutch temperature Tc.

In a step S37, a determination is made as to whether or not the PYDOWNshift predicted temperature T_(ES) is equal to or greater than the DOWNburnout temperature, or in other words whether or not the PYDOWN shiftpredicted temperature T_(ES) is within a temperature region at or abovethe DOWN burnout temperature. When the PYDOWN shift predictedtemperature T_(ES) is lower than the DOWN burnout temperature, theroutine advances to a step S38, where a PYDOWN shift is performed. Whenthe PYDOWN shift predicted temperature T_(ES) is equal to or greaterthan the DOWN burnout temperature, the routine advances to a step S39,where execution of the determined downshift is prohibited.

When the downshift is determined to be caused by depressing theaccelerator in the step S30, on the other hand, the routine advances toa step S40, where a determination is made as to whether or not anaccelerator opening prior to determining a shift in the step S21 isequal to or smaller than a predetermined opening and whether or not thevariation speed of the accelerator opening is equal to or higher than apredetermined speed. When these conditions are satisfied, the routineadvances to a step S46, and when either one of these conditions is notsatisfied, the routine advances to a step S41. The predetermined openingis set at substantially zero, and the predetermined speed is set at asufficient value for determining rapid depression of an acceleratorpedal. In other words, these conditions are established when rapiddepression is performed from a state in which the accelerator opening issubstantially fully closed. This case corresponds to a case in whichfirst synchronization control is performed, and therefore the routineadvances to the step S46. A case in which the above conditions are notestablished corresponds to a case in which second synchronizationcontrol is performed, and therefore the routine advances to the stepS41.

The first synchronization control and second synchronization control arecontrol to engage the clutch to be engaged after synchronizing theengine rotation speed and the rotation speed of the clutch during adownshift. In the first synchronization control, the disengagement sideclutch is disengaged rapidly without dragging the clutch, or in otherwords, the oil pressure supplied to the clutch is reduced stepwise. Inthe second synchronization control, the clutch is disengaged while beingdragged, or in other words the oil pressure supplied to the clutch isreduced gradually, with the aim of eliminating a sense of output torqueloss.

In the step S41, a second synchronized shift predicted temperatureincrease T_(INH) is calculated. The second synchronized shift predictedtemperature increase is the predicted temperature increase T_(INH) ofthe clutch that is disengaged during a shift performed in accordancewith the second synchronization control, and a calculation methodthereof will be described in detail below.

In a step S42, a second synchronized shift predicted temperature T_(ES)is determined by adding the second synchronized shift predictedtemperature increase T_(INH) to the current clutch temperature Tc.

In a step S43, a determination is made as to whether or not the secondsynchronized shift predicted temperature T_(ES) is equal to or greaterthan the DOWN burnout temperature. When the second synchronized shiftpredicted temperature T_(ES) is lower than the DOWN burnout temperature,the routine advances to a step S44, where a shift corresponding to thesecond synchronization control is performed. When the secondsynchronized shift predicted temperature T_(ES) is equal to or greaterthan the DOWN burnout temperature, the routine advances to a step S45,where execution of the determined downshift is prohibited.

When it is determined in the step S40 that the accelerator opening priorto determination of a shift command is equal to or smaller than thepredetermined opening and the variation speed of the accelerator openingis equal to or greater than the predetermined speed, on the other hand,the routine advances to the step S46, where the current clutchtemperature Tc is read.

In a step S47, a determination is made as to whether or not the currentclutch temperature Tc is equal to or greater than the DOWN burnouttemperature. If the current clutch temperature Tc is lower than the DOWNburnout temperature, the routine advances to a step S48, where a shiftcorresponding to the first synchronization control is performed, andwhen the current clutch temperature Tc is equal to or greater than theDOWN burnout temperature, the routine advances to a step S49, where thedownshift is prohibited.

When it is determined in the step S22 that the determined shift is achange-mind shift, the routine advances to the step S50 in FIG. 14,where a determination is made as to whether the shift is an upshift or adownshift. When an upshift is determined, the routine advances to a stepS51, and when a downshift is determined, the routine advances to a stepS57. In the step S50, similarly to the step S23, an upshift onlysignifies an engagement transition state upshift and a downshift onlysignifies a disengagement transition state downshift.

In the step S51, the current clutch temperature Tc is read.

In a step S52, the continuous change-mind shift allowable numbercorresponding to the clutch temperature Tc during an UP shift is read.The continuous change-mind shift allowable number is determined on thebasis of the clutch temperature Tc by referring to a map shown in FIG.15.

The map shown in FIG. 15 is divided into four regions corresponding tothe clutch temperature Tc, namely an S region, an A region, a B region,and a C region. The change-mind shift allowable number is determinedaccording to the region in which the current clutch temperature Tc islocated. In the S region, the clutch temperature Tc is equal to orhigher than the UP burnout temperature. In the A region, the clutchtemperature Tc is lower than the UP burnout temperature and equal to orhigher than the DOWN burnout temperature. In the B region, the clutchtemperature Tc is lower than the DOWN burnout temperature and equal toor higher than a temperature obtained by subtracting an upshift periodmaximum heat generation amount T_(up) from the UP burnout temperature.In the C region, the clutch temperature Tc is lower than the temperatureobtained by subtracting the upshift period maximum heat generationamount T_(up) from the UP burnout temperature.

When the current clutch temperature Tc is in the S region, clutch burnoccurs, and therefore a change-mind shift is prohibited and thecontinuous change-mind shift allowable number is set at zero. When thecurrent clutch temperature Tc is in the A region, a single change-mindshift may cause the clutch temperature Tc to enter the S region, andtherefore a change-mind shift is prohibited and the continuouschange-mind shift allowable number is set at zero. When the currentclutch temperature Tc is in the B region, an upshift change-mind shiftperformed during a downshift can restrict a subsequent downshift, andtherefore the continuous change-mind shift allowable number is set atone. When the current clutch temperature Tc is in the C region, there isno need to limit the number of change-mind shifts, but here, thecontinuous change-mind shift allowable number is set at five, forexample.

Returning to FIG. 14, in a step S53, a determination is made as towhether or not the current continuous change-mind shift number issmaller than the continuous change-mind shift allowable number. When thecurrent continuous change-mind shift number is smaller than thecontinuous change-mind shift allowable number, the routine advances to astep S54, where the continuous shift number is incremented. The routinethen advances to a step S55, where an upshift is performed. When thecurrent continuous change-mind shift number is equal to or higher thanthe continuous change-mind shift allowable number, the routine advancesto a step S56, where an upshift is prohibited.

When a downshift is determined in the step S50, on the other hand, theroutine advances to a step S57, where the current clutch temperature Tcis read.

In a step S58, the continuous change-mind shift allowable numbercorresponding to the clutch temperature Tc during a downshift is read.The downshift continuous change-mind shift allowable number isdetermined similarly to the upshift continuous change-mind shiftallowable number determined in the step S52, but differs therefrom whenthe clutch temperature Tc is in the B region. A downshift change-mindshift performed during an upshift may lead subsequently to a forcibleupshift to prevent over-revving of the engine, and in consideration ofthis upshift, a change-mind shift is prohibited.

In a step S59, a determination is made as to whether or not the currentcontinuous change-mind shift number is smaller than the continuouschange-mind shift allowable number. When the current continuouschange-mind shift number is smaller than the continuous change-mindshift allowable number, the routine advances to a step S60, where thecontinuous shift number is incremented. The routine then advances to astep S61, where a downshift is performed. When the current continuouschange-mind shift number is equal to or higher than the continuouschange-mind shift allowable number, the routine advances to a step S62,where a downshift is prohibited.

Next, calculation of the UP shift predicted temperature increase T_(INH)in the step S24 of FIG. 13 will be described with reference to theflowchart in FIG. 16 and the time chart in FIG. 20. The time chart inFIG. 20 shows (a) a target gear position NxtGP, (b) a current gearposition CurGP, (c) the turbine rotation speed NT, (d) the outputrotation speed No (vehicle speed), (e) acceleration, (f) the relativerotation speed, (g) the transmission torque of the clutch, and (h)variation in the oil pressure supplied to the clutch. A time periodt1-t2 is a pre-processing period, a time period t2-t3 is a torque phasetarget period, and a time period t3-t4 is an inertia phase targetperiod. Pre-processing corresponds to the time period extending from theshift command to completion of a piston stroke of the clutch.

In a step S201, the acceleration at the start of pre-processing ((e) inFIG. 20; t1) is calculated. The acceleration at the start ofpre-processing is calculated on the basis of the vehicle speed at thestart of pre-processing and the vehicle speed at a predeterminedprevious time.

In a step S202, the pre-processing period (t2-t1) is read. Thepre-processing period is determined on the basis of the vehicle speedand torque, and in this embodiment, a pre-processing period backup timerprovided for shift control is read.

In a step S203, the vehicle speed at the start of the torque phase ((d)in FIG. 20; t2) is calculated. The vehicle speed at the start of thetorque phase is calculated by adding a value obtained by multiplying thepre-processing period by the acceleration at the start of pre-processingto the vehicle speed at the start of pre-processing.

In a step S204, turbine torque at the start of the torque phase iscalculated. The turbine torque at the start of the torque phase iscalculated by referring to a pre-stored rotation-torque conversion mapon the basis of the turbine rotation speed NT, which is determined fromthe vehicle speed at the start of the torque phase and the gear ratio.

In a step S205, the torque phase target period (t3-t2) of the shiftcontrol is read on the basis of the vehicle speed at the start of thetorque phase and the turbine torque.

In a step S206, transmission torque at the start of the torque phase((g) in FIG. 20; t2) is calculated. The transmission torque at the startof the torque phase counterbalances a return spring of the clutch, andsince no oil pressure is supplied at the start of the torque phase, thetransmission torque at the start of the torque phase is zero.

In a step S207, the vehicle speed at the start of the inertia phase ((d)in FIG. 20; t3) is calculated. The vehicle speed at the start of theinertia phase is calculated by adding the vehicle speed at the start ofthe torque phase to a value obtained by multiplying the torque phasetarget period by the acceleration at the start of pre-processing.

In a step S208, the turbine torque at the start of the inertia phase iscalculated. The turbine torque at the start of the inertia phase iscalculated by referring to the rotation-torque conversion map on thebasis of the turbine rotation speed NT, which is determined from thevehicle speed at the start of the inertia phase and the gear ratio.

In a step S209, the transmission torque at the start of the inertiaphase ((g) in FIG. 20; t3) is calculated. The transmission torque at thestart of the inertia phase is calculated by multiplying an apportionmentratio by the turbine torque at the start of the inertia phase. Theapportionment ratio is a ratio between the torque received by theplurality of engaged clutches in a certain gear position and the inputtorque.

In a step S210, an average torque phase transmission torque ((g) in FIG.20) is calculated. The average torque phase transmission torque iscalculated by halving a value obtained by adding the transmission torqueat the start of the inertia phase to the transmission torque at thestart of the torque phase. In other words, the average torque phasetransmission torque is calculated as an average value of thetransmission torque at the start of the torque phase and thetransmission torque at the start of the inertia phase.

In a step S211, the oil pressure at the start of the inertia phase ((h)in FIG. 20; t2) is calculated. The oil pressure at the start of theinertia phase is calculated in accordance with the following Equation(8).(oil pressure at start of inertia phase)=(transmission torque at startof inertia phase)/(A×μ×D×N)+F/A  (8)

Here, A is a surface area, μ is a frictional coefficient, D is aneffective diameter, N is a facing number, and F is the load of thereturn spring.

In a step S212, an oil pressure incline at the start of the inertiaphase is read from the shift control map on the basis of the turbinetorque at the start of the inertia phase and the vehicle speed at thestart of the inertia phase.

In a step S213, an average inertia phase oil pressure is calculated. Theaverage inertia phase oil pressure is calculated on the basis of the oilpressure at the start of the inertia phase, the oil pressure incline atthe start of the inertia phase, and the inertia phase target period. Itshould be noted that the inertia phase target period is a constant.

In a step S214, an average inertia phase transmission torque ((g) inFIG. 20) is calculated on the basis of the average inertia phase oilpressure.

In a step S215, the relative rotation speed at the start of the torquephase ((f) in FIG. 20; t2) is calculated. The relative rotation speed atthe start of the torque phase is calculated in accordance with thefollowing Equation (9).(relative rotation speed at start of torque phase)={A×(output rotationspeed No at start of torque phase)+B×(turbine rotation speed NT at startof torque phase)}×2π/60  (9)

Here, A and B are relative rotation calculation constants determined inadvance from a collinear graph.

In a step S216, the relative rotation speed at the start of the inertiaphase ((f in FIG. 20; t3) is calculated. The relative rotation speed atthe start of the inertia phase is calculated in accordance with thefollowing Equation (10).(relative rotation speed at start of inertia phase)={A×(output rotationspeed No at start of inertia phase)+B×(turbine rotation speed NT atstart of inertia phase)}×2π/60  (10)

In a step S217, an average torque phase relative rotation speed ((f) inFIG. 20) is calculated. The average torque phase relative rotation speedis calculated by halving a value obtained by adding the relativerotation speed at the start of the inertia phase to the relativerotation speed at the start of the torque phase. In other words, theaverage torque phase relative rotation speed is calculated as an averagevalue of the relative rotation speed at the start of the torque phaseand the relative rotation speed at the start of the inertia phase.

In a step S218, an average inertia phase relative rotation speed ((f) inFIG. 20) is calculated. The average inertia phase relative rotationspeed is calculated by halving the relative rotation speed at the startof the inertia phase. At the end of the inertia phase, the relativerotation speed is zero, and therefore the average inertia phase relativerotation speed is calculated as an average value of the relativerotation speed at the start and end of the inertia phase by halving therelative rotation speed at the start of the inertia phase.

In a step S219, the heat generation amount T_(up) is calculated. Theheat generation amount T_(up) is calculated in accordance with thefollowing Equation (11).(heat generation amount T _(up))={(torque phase period)×(average torquephase relative rotation speed)×(average torque phase transmissiontorque)+(inertia phase period)×(average inertia phase relative rotationspeed)×(average inertia phase transmission torque)}/1000×(Q-T conversioncoefficient)  (11)

Here, the Q-T conversion coefficient is a coefficient for converting[J], which is the unit obtained through the multiplication of time, therelative rotation speed, and torque, into [° C.]. During unitconversion, the coefficient is applied after conversion to [kJ], and istherefore divided by 1000 in advance.

Next, calculation of the DOWN burnout temperature in the step S29 ofFIG. 13 will be described with reference to the flowchart in FIG. 17.

In a step S301, the vehicle speed following a shift to the n−1^(th)speed is calculated.

In a step S302, the acceleration following a shift to the n−1^(th) speedis calculated. The acceleration is calculated on the basis of theturbine torque, which is determined by referring to the rotation-torqueconversion map, after determining the turbine rotation speed NT from thevehicle speed determined in the step S301.

In a step S303, the shift vehicle speed from the n−1^(th) speed to then^(th) speed is calculated. The shift vehicle speed from the n−1^(th)speed to the n^(th) speed is the vehicle speed when an UP shift to then^(th) speed is determined, and is calculated by referring to the shiftmap.

In a step S304, a shift vehicle speed achievement time from the n−1^(th)speed to the n^(th) speed is calculated. The shift vehicle speedachievement time from the n−1^(th) speed to the n^(th) speed iscalculated on the basis of the acceleration calculated in the step S302.

In a step S305, a heat radiation coefficient is calculated. The heatradiation coefficient is calculated on the basis of the heat generationamount T_(up) generated by a downshift and the current clutchtemperature Tc, and is set to increase as the temperature followingcompletion of a downshift rises.

In a step S306, the heat radiation amount T_(down) up to achievement ofthe shift vehicle speed from the n−1^(th) speed to the n^(th) speed iscalculated. The heat radiation amount T_(down) is calculated bymultiplying the shift vehicle speed achievement time from the n−1^(th)speed to the n^(th) speed by the heat radiation coefficient.

In a step S307, a DOWN burnout temperature is calculated. The DOWNburnout temperature is calculated as the lower value of the UP burnouttemperature and a value obtained by adding a temperature reductionresulting from the heat radiation amount T_(down) up to achievement ofthe shift vehicle speed from the n−1^(th) speed to the n^(th) speed to abase DOWN burnout temperature.

Here, calculation of the normal DOWN shift predicted temperatureincrease T_(INH) in the step S31 of FIG. 13 will be described withreference to the flowchart in FIG. 18 and the time chart in FIG. 21. Thetime chart in FIG. 21 shows (a) the turbine rotation speed NT, (b) theoutput rotation speed No (vehicle speed), (c) acceleration, (d) therelative rotation speed, and (e) variation in the transmission torque ofthe clutch. The time period t1 to t2 is the inertia phase target period.

In a step S401, the vehicle speed at the start of the inertia phase ((b)in FIG. 21; t1) is calculated. The vehicle speed at the start of theinertia phase is calculated by adding the vehicle speed at the start ofpre-processing to a value obtained by multiplying the pre-processingperiod by the acceleration at the start of pre-processing.

In a step S402, the turbine torque at the start of the inertia phase iscalculated by referring to the rotation-torque conversion map on thebasis of the turbine rotation speed NT, which is determined from thevehicle speed at the start of the inertia phase and the gear ratio.

In a step S403, the transmission torque at the start of the inertiaphase ((e) in FIG. 21; t1) is calculated. The transmission torque at thestart of the inertia phase is calculated by multiplying theapportionment ratio by the turbine torque at the start of the inertiaphase.

In a step S404, the vehicle speed at the end of the inertia phase ((b)in FIG. 21; t2) is calculated. The vehicle speed at the end of theinertia phase is calculated on the basis of the current acceleration,the pre-processing period, and the inertia phase target period.

In a step S405, the turbine torque at the end of the inertia phase iscalculated. The turbine torque at the end of the inertia phase iscalculated by referring to the rotation-torque conversion map on thebasis of the turbine rotation speed NT, which is determined from thevehicle speed at the end of the inertia phase and the gear ratio.

In a step S406, the transmission torque at the end of the inertia phase((e) in FIG. 21; t2) is calculated. The transmission torque at the endof the inertia phase is calculated by multiplying the apportionmentratio and a safety factor by the turbine torque at the end of theinertia phase. The safety factor is a constant for determining the oilpressure upon disengagement of the clutch during a downshift, and isdetermined on the basis of the turbine torque at the end of the inertiaphase and the vehicle speed.

In a step S407, the average inertia phase transmission torque ((e) inFIG. 21) is calculated. The average inertia phase transmission torque iscalculated by halving a value obtained by adding the transmission torqueat the end of the inertia phase to the transmission torque at the startof the inertia phase. In other words, the average inertia phasetransmission torque is calculated as an average value of thetransmission torque at the start of the inertia phase and thetransmission torque at the end of the inertia phase.

In a step S408, an average inertia phase relative rotation speed ((d) inFIG. 21) is calculated. The average inertia phase relative rotationspeed is calculated in accordance with the following Equation (12).(average inertia phase relative rotation speed)={A×(output rotationspeed No at start of inertia phase)+B×(turbine rotation speed NT atstart of inertia phase)}×π/60  (12)

Here, A and B are relative rotation calculation constants determined inadvance from a collinear graph.

In a step S409, the heat generation amount T_(up) is calculated. Theheat generation amount T_(up) is calculated in accordance with thefollowing Equation (13).(heat generation amount T _(up))={(inertia phase period)×(averageinertia phase relative rotation speed)×(average inertia phasetransmission torque)}/1000×(Q-T conversion coefficient)  (13)

Calculation of the PYDOWN shift predicted temperature increase T_(INH)in the step S35 of FIG. 13 is similar to calculation of the normal DOWNshift predicted temperature increase T_(INH) described above, butdiffers therefrom in that the inertia phase target period used in thestep S404 is shorter than that of the normal DOWN shift.

Next, calculation of the second synchronized shift predicted temperatureincrease T_(INH) in the step S41 of FIG. 13 will be described withreference to the flowchart in FIG. 19.

In a step S501, the relative rotation speed between the turbine rotationspeed NT and the output rotation speed No is calculated.

In a step S502, a target transmission torque of the clutch to bedisengaged is calculated.

In a step S503, a target shift period is calculated.

In a step S504, a predicted heat generation amount T_(up) is calculated.The predicted heat generation amount T_(up) is calculated by multiplyingthe relative rotation speed, the target transmission torque, and thetarget shift period.

Next, actions of the shift control device for an automatic transmissionaccording to this embodiment will be described with reference to thetime chart shown in FIG. 22. Unless otherwise indicated, the termsupshift and downshift are assumed to denote shifts performed in thenormal shift mode, focusing on shift shock. FIG. 22 is a time chartshowing temperature variation in a certain clutch, and illustrates astate in which upshifts and downshifts are repeated between the n^(th)speed and the n+1^(th) speed and heat radiation is performed after eachshift.

When an UP shift command is issued at a time t1, the UP shift predictedtemperature increase T_(INH) is calculated, and since the predictedtemperature T_(ES) following an UP shift, which is obtained by addingthe current clutch temperature Tc to the UP shift predicted temperatureincrease T_(INH), does not exceed the UP burnout temperature, an upshiftis performed.

When a downshift command is issued at a time t2, the DOWN shiftpredicted temperature increase T_(INH) is calculated, and since thepredicted temperature T_(ES) following a downshift, which is obtained byadding the current clutch temperature Tc to the DOWN shift predictedtemperature increase T_(INH), does not exceed the DOWN burnouttemperature, a downshift is performed.

Upshifts and downshifts are repeated in a similar manner thereafter, andwhen an upshift is determined at a time t3, the predicted temperatureT_(ES) following the upshift is calculated. Since this predictedtemperature T_(ES) exceeds the UP burnout temperature, a PYUP shiftexecuted in the low heat generation amount is performed. As a result,the heat generation amount T_(up) of the clutch decreases, and thereforeburnout caused when the clutch temperature exceeds the UP burnouttemperature is avoided.

Thereafter, the clutch enters the engagement steady state and graduallyradiates heat. The heat radiation amount T_(down) at this time, or inother words the temperature decrease gradient, is determined on thebasis of the temperature difference between the clutch temperatureimmediately after an upshift performed from the time t3 onward and theoil temperature T_(OIL).

When a downshift is determined at a time t4, the predicted temperatureT_(ES) following a downshift executed in the normal shift mode iscalculated, and since this predicted temperature T_(ES) exceeds the DOWNburnout temperature, the predicted temperature T_(ES) following a PYDOWNshift executed in the low heat generation amount shift mode iscalculated. However, the predicted temperature T_(ES) following a PYDOWNshift also exceeds the DOWN burnout temperature, and therefore executionof the determined downshift is prohibited.

When a downshift is determined again at a time t5, the predictedtemperature T_(ES) following a downshift executed in the normal shiftmode is calculated, and since this predicted temperature T_(ES) exceedsthe DOWN burnout temperature, the predicted temperature T_(ES) followinga PYDOWN shift is calculated. In this case, the predicted temperatureT_(ES) following a PYDOWN shift executed in the low heat generationamount shift mode does not exceed the DOWN burnout temperature, andtherefore a PYDOWN shift is performed.

Thereafter, the clutch enters the disengagement steady state andgradually radiates heat. The heat radiation amount T_(down) at thistime, or in other words the temperature decrease gradient, is determinedon the basis of the temperature difference between the clutchtemperature immediately after a downshift performed from the time t5onward and the oil temperature T_(OIL).

When the clutch resetting set time period elapses after the time t5, orthe clutch temperature falls to or below the oil temperature T_(OIL),the clutch temperature is held at the oil temperature T_(OIL) (a fixedvalue).

In the embodiment described above, the heat generation amount T_(up)produced by a shift and the clutch temperature Tc upon completion of theshift are predicted before the start of the shift, and on the basis ofthe predicted clutch temperature Tc, either a normal shift or a PY shiftis determined. In so doing, the shift tolerance can be increased, anddeterioration of the drivability can be prevented. Further, when thepredicted clutch temperature Tc exceeds a burnout temperature, a PYUPshift or a PYDOWN shift resulting in a smaller heat generation amountT_(up) is performed. As a result, shifts that cannot be prohibited, suchas an upshift for preventing over-revving, for example, can be executedin such a manner that the clutch does not burn.

Further, the time required to complete a PYUP shift is shorter than ashift performed in the normal shift mode at the same input torque, andtherefore a corresponding reduction in the heat generation amount T_(up)can be achieved, thereby suppressing reductions in the durability of theclutch.

Furthermore, when a PYUP shift is executed, the torque reduction amountof the engine torque is increased beyond that of an upshift performed inthe normal shift mode. As a result, the heat generation amount T_(up)can be reduced correspondingly, thereby suppressing reductions in thedurability of the clutch and increases in shift shock.

In the case of a downshift, when the predicted clutch temperaturefollowing the shift is equal to or greater than the DOWN burnouttemperature, the clutch temperature following a PYDOWN shift performedin the low heat generation amount shift mode is predicted. When thepredicted temperature does not exceed the DOWN burnout temperature, aPYDOWN shift is performed. As a result, the downshift tolerance can bemaximized without worsening the durability of the clutch.

Furthermore, when the predicted clutch temperature following completionof a PYDOWN shift exceeds the DOWN burnout temperature, a downshift isprohibited. In so doing, clutch burnout caused by heat generated when anupshift is performed after a downshift can be forestalled, and thereforereductions in the durability of the clutch can be suppressed.

Further, until the timer reaches a predetermined value, the temperaturedecrease gradient during heat radiation is set on the basis of thetemperature difference between the clutch temperature Tc followingcompletion of the shift and the oil temperature T_(OIL), and when thetimer reaches or exceeds the predetermined value, the temperaturedecrease gradient is set at a fixed predetermined gradient, regardlessof the clutch temperature Tc following completion of the shift and theoil temperature T_(OIL). Thus, in a region where the clutch temperatureTc is comparatively high from the beginning of heat radiation to thepoint at which the timer reaches the predetermined value, the estimationprecision of the current temperature is improved such that deteriorationof the drivability can be prevented. Further, after the timer hasreached the predetermined value, the clutch temperature Tc is low andthe temperature decrease gradient may be considered substantiallyconstant, regardless of the clutch temperature Tc at the start of heatradiation. By employing a fixed predetermined gradient, the data volumecan be reduced.

Furthermore, the temperature decrease gradient set on the basis of thetemperature difference between the current clutch temperature Tc and theoil temperature T_(OIL) is set to increase as the clutch temperature Tcupon shift completion rises, and therefore the current clutchtemperature Tc can be calculated with a higher degree of precision.

Furthermore, the predetermined gradient is set at a smaller gradientthan the temperature decrease gradient set on the basis of temperaturedifference between the current clutch temperature Tc and the oiltemperature T_(OIL), and therefore the current clutch temperature Tc canbe calculated with a higher degree of precision.

Further, when the reset determination timer reaches or exceeds theclutch resetting set time period, the clutch temperature Tc is set atthe oil temperature T_(OIL). Here, when a certain amount of time haselapsed following the start of heat radiation, it may be determined thatthe clutch temperature Tc has fallen to a temperature in the vicinity ofthe oil temperature T_(OIL), and therefore, in this case, calculation ofthe clutch temperature Tc is halted, enabling a reduction in thecalculation load.

Further, when the calculated clutch temperature Tc falls to or below theoil temperature T_(OIL), calculation is halted and the clutchtemperature Tc is set at the oil temperature T_(OIL), thereby preventingcalculations according to which the clutch temperature Tc is lower thanthe oil temperature T_(OIL), a result that is impossible in reality.

Further, the heat generation amount T_(up) produced by a shift and theclutch temperature Tc upon completion of the shift are predicted beforethe start of the shift, and on the basis of the predicted clutchtemperature Tc, a shift is either permitted or prohibited. In so doing,the shift tolerance can be increased, and deterioration of thedrivability can be prevented. Further, the heat generation amount T_(up)of the clutch during the shift is predicted on the basis of an averagevalue of the transmission torque of the clutch and an average value ofthe relative rotation speed of the clutch, and therefore an improvementin prediction accuracy can be achieved while suppressing the calculationload in comparison with a case in which prediction is performed byintegrating hydraulic data.

Furthermore, the vehicle speed, the turbine torque, the transmissiontorque of the clutch, and the relative rotation speed are predicted onthe basis of the acceleration before the start of the shift, andmoreover, the average value of the transmission torque of the clutch andthe average value of the relative rotation speed of the clutch are alsopredicted. Hence, the data amount can be reduced, and data setting canbe facilitated.

Moreover, during an upshift, an average value of the transmission torqueof the inertia phase is calculated on the basis of the incline of theoil pressure supplied to the clutch at the start of the inertia phaseand the target time period of the inertia phase, and therefore thecalculation load can be reduced while maintaining prediction accuracy.

This application claims priority from Japanese Patent Application Nos.2007-250245, 2007-250251 and 2007-250261 each were filed on Sep. 26,2007, which are incorporated herein by reference in their entirety.

1. An automatic transmission comprising: a shift mechanism that executesa shift from a current gear position to a target gear position byengaging or disengaging a plurality of frictional elements selectively;a shift control unit which performs the shift in a first shift mode; acurrent thermal load calculating unit which calculates a current thermalload state of a frictional element; a first heat generation amountpredicting unit which predicts, prior to start of the shift, a heatgeneration amount of the frictional element if the shift is performed inthe first shift mode; and a first thermal load predicting unit whichpredicts a thermal load state of the frictional element upon shiftcompletion if the shift is performed in the first shift mode on a basisof the current thermal load state of the frictional element and the heatgeneration amount predicted by the first heat generation amountpredicting unit, wherein the shift control unit either performs theshift in a second shift mode, in which a heat generation amount is lowerthan that of the first shift mode, or prohibits the shift when thethermal load state upon shift completion predicted by the first thermalload predicting unit is inside a predetermined region.
 2. The automatictransmission as defined in claim 1, wherein the shift is an upshift, andwherein a time required to complete the shift in the second shift modeis shorter than that of the first shift mode at an identical inputtorque.
 3. The automatic transmission as defined in claim 2, furthercomprising a drive source torque reduction command outputting unit whichoutputs a command to a drive source to reduce an input torque inputtedinto the transmission from the drive source temporarily during theshift, wherein a reduction amount in the input torque from the drivesource is larger in the second shift mode than in the first shift mode.4. The automatic transmission as defined in claim 1, wherein the shiftis a downshift, wherein the automatic transmission further comprises: asecond heat generation amount predicting unit which predicts, prior tothe start of the shift, the heat generation amount of the frictionalelement if the shift is performed in the second shift mode when thethermal load state upon shift completion predicted by the first thermalload predicting unit is inside the predetermined region; and a secondthermal load predicting unit which predicts the thermal load state ofthe frictional element upon shift completion if the shift is performedin the second shift mode on a basis of the current thermal load state ofthe frictional element and the heat generation amount predicted by thesecond heat generation amount predicting unit, and wherein the shiftcontrol unit performs the shift in the second shift mode when thethermal load state upon shift completion predicted by the first thermalload predicting unit is inside the predetermined region and the thermalload state upon shift completion predicted by the second thermal loadpredicting unit is outside the predetermined region.
 5. The automatictransmission as defined in claim 4, wherein the shift control unitprohibits the shift when the thermal load state upon shift completionpredicted by the second thermal load predicting unit is inside thepredetermined region.
 6. The automatic transmission as defined in claim1, further comprising an oil temperature detecting unit for detecting anoil temperature of the automatic transmission, wherein the currentthermal load calculating unit calculates the current thermal load stateof the frictional element on a basis of a decrease gradient of thethermal load state of the frictional element and an elapsed timefollowing shift completion, and wherein the decrease gradient is a firstdecrease gradient set on a basis of the thermal load state upon shiftcompletion and the oil temperature from shift completion to an elapse ofa first predetermined time period, and is a constant second decreasegradient regardless of the thermal load state upon shift completion andthe oil temperature once the first predetermined time period has elapsedfollowing shift completion.
 7. The automatic transmission as defined inclaim 6, wherein the first decrease gradient is set at a larger gradientas the thermal load state upon shift completion increases.
 8. Theautomatic transmission as defined in claim 6, wherein the seconddecrease gradient is smaller than the first decrease gradient.
 9. Theautomatic transmission as defined in claim 6, wherein the thermal loadstate is a temperature, and wherein the current thermal load calculatingunit halts calculation of the thermal load state of the frictionalelement and sets the current thermal load state of the frictionalelement at a current oil temperature when a second predetermined timeperiod, which is longer than the first predetermined time period,elapses following shift completion.
 10. The automatic transmission asdefined in claim 6, wherein the thermal load state is a temperature, andwherein the current thermal load calculating unit halts calculation ofthe thermal load state of the frictional element and sets the currentthermal load state of the frictional element at a current oiltemperature when the current thermal load state of the frictionalelement calculated by the current thermal load calculating unit falls toor below the oil temperature.
 11. The automatic transmission as definedin claim 1, wherein the first heat generation amount predicting unitpredicts the heat generation amount of the frictional element on a basisof an average value of a transmission torque of the frictional elementand an average value of a relative rotation speed of the frictionalelement during the shift.
 12. The automatic transmission as defined inclaim 11, further comprising: a vehicle speed predicting unit whichpredicts a vehicle speed at a start of a torque phase and a vehiclespeed at a start of an inertia phase on a basis of an acceleration priorto the start of the shift; a turbine torque predicting unit whichpredicts a turbine torque at the start of the torque phase and a turbinetorque at the start of the inertia phase on a basis of the vehicle speedat the start of the torque phase and the vehicle speed at the start ofthe inertia phase; a transmission torque predicting unit which predictsa transmission torque of the frictional element at the start of thetorque phase and a transmission torque of the frictional element at thestart of the inertia phase on a basis of the turbine torque at the startof the torque phase and the turbine torque at the start of the inertiaphase; and a relative rotation speed predicting unit which predicts arelative rotation speed of the frictional element at the start of thetorque phase and a relative rotation speed of the frictional element atthe start of the inertia phase on a basis of the vehicle speed at thestart of the torque phase and the vehicle speed at the start of theinertia phase, wherein the average value of the transmission torque ofthe frictional element during the shift is calculated on a basis of thetransmission torque of the frictional element at the start of the torquephase and the transmission torque of the frictional element at the startof the inertia phase, and wherein the average value of the relativerotation speed of the frictional element is calculated on a basis of therelative rotation speed of the frictional element at the start of thetorque phase and the relative rotation speed of the frictional elementat the start of the inertia phase.
 13. The automatic transmission asdefined in claim 11, wherein, when the shift is an upshift, the averagevalue of the transmission torque of the inertia phase is calculated on abasis of an incline of oil pressure supplied to the frictional elementat a start of an inertia phase and a target time period of the inertiaphase.
 14. A shift control method for an automatic transmission thatexecutes a shift from a current gear position to a target gear positionby engaging or disengaging a plurality of frictional elementsselectively, wherein the method comprises: performing the shift in afirst shift mode; calculating a current thermal load state of africtional element; predicting, prior to a start of the shift, a heatgeneration amount of the frictional element if the shift is performed inthe first shift mode; and predicting a thermal load state of thefrictional element upon shift completion if the shift is performed inthe first shift mode on a basis of the current thermal load state of thefrictional element and the predicted heat generation amount; andperforming the shift in a second shift mode, in which a heat generationamount is lower than that of the first shift mode, or prohibiting theshift when the predicted thermal load state is inside a predeterminedregion.
 15. An automatic transmission comprising: a shift mechanism thatexecutes a shift from a current gear position to a target gear positionby engaging or disengaging a plurality of frictional elementsselectively; shift control means for performing the shift in a firstshift mode; current thermal load calculating means for calculating acurrent thermal load state of a frictional element; first heatgeneration amount predicting means for predicting, prior to a start ofthe shift, a heat generation amount of the frictional element if theshift is performed in the first shift mode; and first thermal loadpredicting means for predicting a thermal load state of the frictionalelement upon shift completion if the shift is performed in the firstshift mode on a basis of the current thermal load state of thefrictional element and the heat generation amount predicted by the firstheat generation amount predicting means, wherein the shift control meanseither performs the shift in a second shift mode, in which a heatgeneration amount is lower than that of the first shift mode, orprohibits the shift when the thermal load state upon shift completionpredicted by the first thermal load predicting means is inside apredetermined region.